1. Field of the Invention
The present invention relates to semiconductor laser devices using a compound semiconductor material and, more particularly, to a method of manufacturing semiconductor laser devices for stably providing a laser oscillation of visible light at room temperature.
2. Description of the Related Art
Rapid advances in semiconductor technologies have led to a variety of applications for semiconductor devices serving as solid-state light-emitting elements. Unceasing efforts have been made to satisfy needs for higher outputs and higher reliability. In recent years, indium/gallium/aluminum/phosphorus (to be referred to as "InGaAlP" hereinafter)-based semiconductor materials have received a great deal of attention as materials for semiconductor lasers which provide visible light oscillation. This is because the compound semiconductor materials have largest band gaps among Group III-V compound semiconductor mixed crystals except for nitrides.
In particular, a semiconductor laser with a double-heterostructure having active and cladding layer of InGaAlP on a gallium arsenide (to be referred to as "GaAs" hereinafter) substrate can exhibit a stable laser oscillation of visible light in a 0.6-micrometer band at room temperature. A semiconductor laser of this type is expected to be used in various applications which cannot be realized using conventional infrared-range semiconductor lasers. Therefore, a semiconductor laser of this type is a very expecting light-emitting device.
Presently available semiconductor laser devices having an InGaAlP double-heterostructure, however, suffer from the following disadvantages: (1) a light output cannot be increased up to a desired value; and (2) it is difficult to keep satisfactory operation reliability in a maximum light output oscillation mode.
According to the studies by the present inventors, it was confirmed that the above problems were mainly caused by undesired self light absorption at a light output end of an InGaAlP active layer in a laser oscillation mode, as will be explained hereinafter in more detail.
In general, a stripe light guide structure is applied to an InGaAlP semiconductor laser. A GaAs current-blocking layer is formed on a double-heterostructure in which an InGaAlP active layer is sandwiched by upper and lower cladding layers, and the current-blocking layer is provided with an elongated groove, i.e., a stripe opening. A GaAs ohmic contact layer covers the channel and the current-blocking layer. In an oscillation mode, oscillated laser beams are confined in only the opening, and hence this opening serves as a light waveguide channel. An oscillation wavelength is determined depending on a band gap energy of the InGaAlP layer serving as a light-emitting region of the active layer. An InGaAlP active layer is conventionally formed using an epitaxial growth method which is popular among persons skilled in the art. Therefore, a band gap energy is simply constant across the entire active layer.
With such an arrangement, when supply of a maximum injection current is continued in order to successively perform an oscillation at a maximum light output level of the semiconductor laser at room temperature, the light output is abruptly decreased, and operation stability is damaged. Such degradation in performance is also observed in semiconductor lasers of other types such as a transverse mode stabilized InGaAlP laser. Therefore, the above-mentioned degradation in performance is an inherent phenomenon of InGaAlP compound semiconductor materials, and can be considered to be caused by its limited inherent allowable light density. In practice, substrates of several elements degraded due to laser oscillation were removed, and a current injection light-emitting pattern was observed from the substrate side. It was confirmed that a black portion was formed near each light output end of the laser element. This observation result demonstrates generation of the following vicious circle. That is, a light density of the laser beam output end of the InGaAlP active layer is increased beyond an inherent allowable limit of the material, self light absorption is accelerated, heat is generated, and the heat generation causes further acceleration of self light absorption. Such a vicious circle of self light absorption adversely affects, i.e., not only decreases a laser oscillation efficiency, but also physically breaks down a light output layer in the active layer, since it causes a damage, melting, and degradation of the layer quality of the layers near the laser oscillation output end in the active layer.
These problems have spoiled usefulness and future applications of InGaAlP semiconductor laser devices and are decisive for semiconductor manufacturers. Applications of InGaAlP laser devices without drastic resolutions for the above problems seem impossible.
In order to prevent degradation in performance associated with the above-mentioned self light absorption, several high-output semiconductor laser devices each having a "non-absorbing mirror" structure have been proposed. For example, a self-absorption prevention technique is disclosed in "An AlGaAs Window Structure Laser", HIRO O. YONEZU et al, IEEE Journal of Quantum Electronics, Vol. QE-15, No. 8, August, 1979 at pp. 775-781, wherein impurity such as Zn is doped into an active layer in such a manner that the both end portions thereof are kept undoped, so that the band gap energy at the both end portion of the active layer is higher than that of the remaining portions thereof. Another technique is disclosed in "AlGaAs Window Stripe Buried Multiquantum Well Lasers", HISAO NAKASHIMA et al, Japanese Journal of Applied Physics, Vol. 24, No. 8, August, 1985, at pp. L647-L649, wherein an impurity such as Zn is diffused in two end portions of the active layer to increase its internal band gap energy in only the diffusion region, thus decreasing self light absorption. According to the technique disclosed in this article, however, a method of manufacturing lasers with a "non-absorbing mirror" structure requires very complicated manufacturing processes involving strict control of crystal growth. Therefore, practical application of these processes cannot be expected for semiconductor manufacturers very much. In addition, application of these techniques to the manufacture of high-power semiconductor lasers using a specific InGaAlP-based semiconductor material, which suffers from more difficult crystal growth manufacture than GaAlAs, can be hardly expected.